In semiconductor manufacturing, a semiconductor workpiece or wafer often undergoes many processing steps or stages before a completed die is formed. For example, lithographic processes are performed on the semiconductor wafer using a mask and photoresist to transfer a particular design or layout onto the wafer. Design Rules (DRs) are a series of parameters provided by semiconductor manufacturers that enable a designer to verify the correctness of a mask or mask set. Design rules are often specific to a particular semiconductor manufacturing process. A design rule set specifies certain geometric and connectivity restrictions to, among other things, ensure sufficient margins to account for variability in semiconductor manufacturing processes, so as to ensure that most of the resultant components work as designed.
As integrated circuits become smaller, they are designed with ever-decreasing feature dimensions. Accordingly, lithographic processes performed on the semiconductor wafer play an important role in the continued reduction in feature dimensions. For example, the fabrication of integrated circuits having line widths smaller than 0.18 μm largely depends on the continued development of photolithography. In order to reduce the size of semiconductor devices, the resolution of a photomask in photolithographic processing needs to be increased. Developments such as optical proximity correction (OPC) and phase shift masks (PSMs) have been introduced in order to continue the reduction in feature dimensions.
Conventionally, optical proximity correction is one of the primary methods in attempting to maintain critical dimensions which can deviate due to proximity effects. Proximity effects can occur during lithographic processing when a beam of light passes through a photomask, wherein the beam of light projects through a pattern on the photomask to expose a photoresist on a surface of the semiconductor wafer to the pattern. During the exposure of the wafer to the beam of light, light rays may be diffracted by the photomask such that a portion of the light rays diverge. Further, some of the light passing into the photoresist can be reflected by the semiconductor substrate of the workpiece, thus causing interference. As such, a portion of the photoresist layer may be repeatedly exposed to the beam of light, thus leading to undesirable variation in photoresist exposure.
One such undesirable variation is an effect called corner rounding, where substantially square corners of a pattern are rounded in the resulting exposed photoresist. FIG. 1A illustrates a top view of a conventional photomask 100 with a pattern 105 for manufacturing a semiconductor circuit. FIG. 1B illustrates a top view of the resulting pattern 110 on a photoresist 115 on a semiconductor wafer 120 after photolithographic processing utilizing the photomask 100 of in FIG. 1A.
After being exposed to light during the photolithography process, a corner 125 of the pattern 105 of FIG. 1A yields a rounded corner 130 of FIG. 1B on the resulting pattern 110 of the photoresist 115, wherein the rounded corner is caused by the above-mentioned proximity effects. Such a rounded corner 125 can deleteriously encroach into a device region 135, as illustrated in region 140, wherein active devices are formed. To prevent such corner rounding from impacting the device region 135, a distance 145 between the active region 135 and the pattern 110 of FIG. 1A is uniformly increased, thus yielding a modified pattern 150 (illustrated in dashed line) of FIG. 1C having a modified distance 155 between the device region 135 and the modified pattern. As seen in FIG. 1C, it is conventional for the modified distance 155 to be uniformly increased in both the x-direction and y-direction, wherein the active region 135 is equally separated from the modified pattern.
However, as device dimensions continue to shrink, lines widths become increasingly narrow, where there is limited area around the device region 135 for forming the modified pattern 150 while meeting minimum design rule criteria.